High burning rate propellants have been formulated with very fine oxidizer particle sizes; however, the very fine particle sizes (less than 20 microns) result in decreased usable pot life. The usable pot life is the time available for accomplishing the processing steps of mixing and casting propellant before the propellant loses its fluid nature.
Earlier work in propellant technology has resulted in development of reaction products formed from reacting di- or tri-functional aziridinyl phosphine oxides or their derivatives with polyfunctional carboxylic acids. These reaction products have been used to coat inorganic oxidizer particles which are used in propellant compositions, and when so used, the coated particles enhance the mechanical properties of the propellant. U.S. Pat. No. 3,762,972, issuing on Oct. 2, 1973, and its divisional application issuing as U.S. Pat. No. 3,745,074 on July 10, 1973, teach the process of preparation, the products produced, the products coated on ammonium perchlorate, and the coated ammonium perchlorate in propellant compositions. When the propellant compositions containing the described coated ammonium perchlorate are cured, the mechanical properties, particularly the strain at break and tensile strength, are greatly improved at 77.degree.F as well as at -40.degree.F. The propellant compositions contained, generally, a 50--50 blend of very fine particle size ammonium perchlorate (17 micron nominal diameter) and larger particle size ammonium perchlorate (200 micron nominal diameter). The processing of this type blend of oxidizer particles can be accomplished without major problems relating to mixing and pot life.
The advantage of using substantially all very fine ammonium perchlorate in propellant compositions has been demonstrated with respect to improved burn rates. The problems encountered when using very fine ammonium perchlorate, however, which relate to mixing and retaining proper pot life, have required much effort to be devoted to determining what can be done to permit using high percentage of fine ammonium perchlorate while retaining proper pot life for mixing and casting the propellant.
In our copending application Ser. No. 385,930, filed July 27, 1973, the problems of reduced pot life of isocyanate curable propellant compositions were substantially improved by the use of the reaction products formed from tri-functional aziridinylphosphine oxides or their derivatives reacted with monofunctional carboxylic acids, the tri-aziridinyl derivatives of triazine, the tri-aziridinyl derivatives of benzenetriacyl, the aziridine compound N-phenethylaziridine, and selected alkyl diaziridine compounds. The compositions are comprised of a binder of hydroxy terminated liquid polymer systems, (e.g. hydroxy terminated polybutadiene and the like) optional selected plasticizers, optional metal fuel, and the inorganic oxidizer, ammonium perchlorate, that is substantially of very fine particle size (less than 20 microns). The compositions can employ a liquid ferrocene burning rate catalyst or fine Fe.sub.2 O.sub.3 burning rate catalyst.
In high rate propellant, very fine uncoated AP is typically used in conjunction with liquid burn rate catalysts which are notorious cure accelerators and detriments to usable pot life. After the discovery of the catalytic effect of fine AP, it was believed that some of the cure catalysis formerly attributed to the burn rate catalysts was in fact derived from the AP or possibly even a synergistic effect between the burn rate catalyst and the AP. A series of propellant mixes was made with and without aziridine AP coatings, and with and without Catocene, a liquid ferrocene burn rate catalyst. Again it was found that the aziridine coating (precoated on the AP in this series) greatly increased the pot life. Very fine Fe.sub.2 O.sub.3 burn rate catalyst has a similar effect on pot life by acting as a cure accelerator. Although pot life has been extended by aziridine coatings, further extension of pot life would be advantageous for certain propellants.
It is desirable to have an additive for propellant compositions which extends the pot life of propellant compositions employing either coated or uncoated ammonium perchlorate and with or without liquid ferrocene burn rate catalysts or fine particle size Fe.sub.2 O.sub.3 burn rate catalyst.
Therefore, an object of this invention is to provide an additive which extends the pot life of an isocyanate curable propellant composition that employs fine ammonium perchlorate precoated with a selected aziridine compound selected from n-phenethylaziridine (PEA), tris 1(2 methylaziridinyl)phosphine oxide (MAPO), or an adduct (MT4) formed from reacting together 2.0 moles of tris 1(2 methylaziridinyl)phosphine oxide, 0.7 mole adipic acid, and 0.3 mole tartaric acid.
Another object of this invention is to provide an additive which extends the pot life of an isocyanate curable propellant composition employing very fine uncoated ammonium perchlorate.
A further object of this invention is to provide an additive which extends the pot life of an isocyanate curable propellant composition which employs a burn rate catalyst.